A significant increase in the number of wireless broadband users has led to a severe spectrum shortage in the conventional cellular bands. The demand for cellular and other mobile or portable data services is expected to grow at a high rate, necessitating orders of magnitude increases in wireless capacity. Millimeter wave (mmW) frequencies above 6 GHz—including 28, 38, 60, and 73 GHz—have been attracting growing attention as a possible candidate for next-generation microcellular networks. For example, the Institute of Electrical and Electronics Engineers (IEEE) has promulgated wireless networking standard IEEE 802.11ad, also known as “WiGig”, that operates in the 60-GHz spectrum. Millimeter-wave bands offer orders of magnitude greater spectrum and also allow for building high dimensional antenna arrays for further gains via beamforming and spatial multiplexing in both handset devices and infrastructure equipment, as discussed in “Millimeter Wave Wireless Communications,” a textbook co-authored by one of the inventors. The FCC in October 2015 proposed rulemaking for the use of 28, 37, 39 and 64-71 GHz bands, the latter band for unlicensed use. Devices utilizing mmW spectrum are available, such as for line of sight backhaul or for home entertainment in Wireless Local area network (WLAN) situations, but they often require careful and/or lengthy set up or are limited in efficacy because of their use of fixed (e.g., non-adaptable), highly directional antennas for backhaul installations, or for their sensitivity to the blockage caused by people or objects for enterprise or home use, or the lack of wireless or wired/fiber infrastructure or high bandwidth infrastructure to allow a vast network of devices, thus often requiring simple short-range, line-of-sight links, within a controlled and static radio frequency propagation environment, such as in a living room or a data center.
As used herein, “radio frequency propagation environment” (or, more simply, “propagation environment” or “environment”) may include, for example, conditions, terrain, structures, objects, impairments, obstacles, etc. (collectively “factors”) that cause signals of a particular radio frequency, or range of radio frequencies (e.g., signals in one or more bands of mmW frequencies), to behave in a particular manner as they encounter such factors between their transmission point(s) and their reception point(s). Such factors may be more or less fixed (“static”) over time; likewise, one or more may be time-variable to individual degrees. Although such factors are not necessarily spatially uniform throughout a particular propagation environment, their range of spatial variation over a particular propagation environment may be less than their ranges of spatial variation over all possible propagation environments. Moreover, as used herein, “multiple” or “a plurality of” environments may include, without limitation: 1) propagation environments whose respective factors are distinct from each other to some degree (e.g., due to lack of proximity); and 2) propagation environments whose respective factors are not necessarily distinct, but that are relatively isolated from each other (e.g., by a barrier that is relatively impermeable to signals of the particular frequency(ies)).
Since a static environment and conditions may be very difficult to achieve in an uncontrolled setting, there is a need to build mmW systems in practical conditions where line-of-sight is not always available. For future mmW wireless networks, it will be important for such systems to operate properly when the signal-to-noise ratios (SNRs) are lower due to mobility and unpredictable obstructed signals, and where devices are interconnected in a cellular-like or networked-like fashion, for many devices to communicate to each other or through hubs, routers, or base stations, such that the communication is beyond just a single lap top and a single monitor, for example.
One of the challenges facing such mmW systems is the much greater degree of scattering of energy and the difficulty of penetrating most building materials at mmW frequencies compared to conventional systems operating at lower UHF/microwave frequencies (see, e.g., T. S. Rappaport, et al., Wideband Millimeter-Wave Propagation Measurements and Channel Models for Future Wireless Communication System Design, IEEE Trans. Comm., September 2015). Another way to measure this challenge is the attenuation of the signal on the exit side of the barrier compared to the entry side. Since a higher portion of energy is reflected or scattered, mmW signals originating outdoors will have much greater difficulty penetrating the outer walls of buildings into the interiors of buildings, with any mmW signals that do pass being attenuated to a large degree. Previous studies (see, e.g., H. Zhao, et al., 28 GHz Millimeter Wave Cellular Communication Measurements for Reflection and Penetration Loss in and around Buildings in New York City, PROC. IEEE INT'L CONF. ON COMMUNICATIONS (ICC), June 2013), have demonstrated that at certain carrier frequencies, certain substances attenuate signals far more than others. For example, at 28 GHz, brick or tinted glass attenuates signals by a factor of 100 to 10,000 (i.e., 20 to 40 dB in power), whereas normal clear glass used inside buildings attenuates signals substantially less, e.g., by 6 dB or less. Similarly, drywall suffers a modest 7-dB penetration loss, brick suffers a 28-dB loss, and other materials suffer losses of 40 dB or more.
Methods and systems to improve coverage and signal level at frequencies above 6 GHz in particularly desired directions through the use directional or adaptive antennas are described in patents owned by co-inventor T. S. Rappaport, such as U.S. Pat. Pub. No. 2013/0328723 and U.S. Pat. No. 8,593,358. Furthermore, wideband repeaters/relays are described in patents owned by co-inventor T. S. Rappaport, such as U.S. Pat. Nos. 8,611,812; 8,331,854; and other related patents. Methods for transferring signals through barriers such as tinted windows are disclosed, for example, in U.S. Pat. Nos. 5,438,338; 5,451,966; 5,471,222;5,589,839; 5,742,255; 6,172,651; 6,295,033; 6,421,020; 6,490,443; 6,686,882; 7,079,722; and RE33743. Other such methods are described in PCT Pub. Nos. WO1994029926A1 and WO1995014354A1.
Nevertheless, the ability to penetrate particular barriers in controlled ways, such as through specific permeable points in a building or between floors of a building, or at various boundaries that are otherwise difficult to penetrate by mmW frequencies is a novel practical problem for future millimeter wave wireless communication networks, because of the smaller wavelength (and thus greater sensitivities to materials, physical dimensions, antenna configurations, coverage ranges, and so on), as well as the fact that the millimeter wave frontier will require greater densification of cell sites to achieve capacity and coverage while utilizing adaptive arrays and relays. Moreover, the need for increased wireless communications capacity, the inadequate amounts of spectrum available at lower frequencies, and the need for user mobility both inside and outside will necessitate overcoming this increased attenuation, reflectivity, and/or scattering in novel ways in order to communicate across the indoor-outdoor barrier at mmW frequencies. Thus, it can be beneficial to address at least some of the issues and problems identified herein above.